Method of heating up a solid polymer electrolyte fuel cell system

Abstract

A method is disclosed to heat a direct methanol fuel cell stack system at start-up without using any external energy source. During the start-up period, the fuel cell stack is at open circuit state so that the fuel cell stack is not connected to any external circuit. The methanol solution introduced at the anode side of the fuel cell stack will diffuse through the proton conductive membrane to the cathode side of the fuel cell stack. The methanol diffused from the anode side will be oxidized at the cathode side by oxygen in the air stream. This oxidation reaction generates heat that heats up the fuel cell stack and the system. The concentration of methanol in the methanol solution can be varied depending on the initial stack temperature. The lower the initial stack temperature, the higher the concentration of the methanol solution required. At an initial stack temperature of −40° C., a solution having 40 wt % methanol is preferred to avoid freezing of the solution. If the initial stack temperature at start-up is above the freezing point of water, the methanol concentration in the solution can be in a range of 0.5 to 25 wt %, with a more preferred concentration range being from 3 to 10 wt % of methanol. The methanol solution and air feed rates can also be varied to control the rate at which heat is generated in the stack.

1. A method of heating a fuel cell, wherein the fuel cell comprises an anode comprising an anode flow field plate, an anode diffusion layer and an anode catalyst layer, a cathode comprising a cathode flow field plate, a cathode diffusion layer and a cathode catalyst layer, and a proton conductive membrane, the method comprising the steps of:
 (a) operating the fuel cell at an open circuit state, wherein the fuel cell is at an initial temperature;  
 (b) feeding at a fuel feed rate aqueous fuel solution to the anode and feeding at an oxidant food rate an oxidant to the cathode;  
 (c) allowing fuel in the fuel solution to diffuse through the proton conductive membrane from the anode to the cathode;  
 (d) oxidizing the fuel at the cathode to generate heat, thereby heating the fuel cell to an intermediate temperature above the initial temperature;  
 (e) varying the oxidant feed rate so as to control the heating of the fuel cell; and  
 (f) connecting an external circuit to the fuel cell thereby ceasing to operate the fuel cell in the open circuit.  
 
   
   
     2. The method of  claim 1  wherein the fuel is methanol. 
   
   
     3. The method of  claim 2  wherein the fuel solution has a concentration of methanol that is the same as or greater than a second concentration of methanol when the fuel cell is operated under normal conditions. 
   
   
     4. The method of  claim 3  wherein the concentration of methanol is in the range of from 0.5 to 25 wt %. 
   
   
     5. The method of  claim 3 , wherein the concentration of methanol is at least 40 wt % when the initial temperature is less than −40° C. 
   
   
     6. The method of  claim 1  wherein the initial temperature is below the freezing point of water and the intermediate temperature is above the freezing point of water. 
   
   
     7. The method of  claim 1  further comprising the step of varying the fuel feed rate so as to control the heating of the fuel cell. 
   
   
     8. The method of  claim 1  wherein the fuel solution is fed from a fuel reservoir and the method further comprising the step of recycling the aqueous fuel solution back to the fuel reservoir. 
   
   
     9. The method of  claim 1  further comprising the step of controlling temperature of the fuel solution fed to the anode. 
   
   
     10. The method of  claim 1  comprising a plurality of fuel cells arranged in a fuel cell stack. 
   
   
     11. The method of  claim 10  wherein the initial temperature is below the freezing point of water and the intermediate temperature is above the freezing point of water. 
   
   
     12. The method of  claim 10  wherein the fuel in methanol. 
   
   
     13. The method of  claim 12  wherein the fuel solution has a concentration of methanol that is the same as or greater than a second concentration of methanol when the fuel cell is operated under normal conditions. 
   
   
     14. The method of  claim 13  wherein the concentration of methanol is in the range of from 0.5 to 25 wt %. 
   
   
     15. The method of  claim 13  wherein the concentration of methanol is at least 40 wt % when the initial temperature is less than −40° C. 
   
   
     16. The method of  claim 10  further comprising the step of controlling temperature of the fuel solution fed to the anode. 
   
   
     17. The method of  claim 10  further comprising the step of varying the fuel feed rate so as to control the heating of the fuel cell. 
   
   
     18. The method of  claim 10  wherein the fuel solution is fed from a fuel reservoir and the method further comprising the step of recycling the aqueous fuel solution back to the fuel reservoir.